Visually impaired Web users generally use programs called screen readers to navigate websites. The software reads the text to them out loud, but to describe images, it reads out alternate text -- more commonly known as "alt text" -- that the site's editor has attached to the image.
Alternative text is text associated with an image that serves the same purpose and conveys the same essential information as the image. In situations where the image is not available to the reader, perhaps because they have turned off images in their web browser or are using a screen reader due to a visual impairment, the alternative text ensures that no information or functionality is lost. Absent or unhelpful alternative text can be a source of frustration for visually impaired users of the Web.
McGill site editors should add alt text to all image on their sites. For instructions, visit McGill's IT Knowledge Base.
Keep essential info in text format
Alt text is useful, but it isn't much help when it comes to pie charts, diagrams, and other images that can't be summed up in a few words. Whenever possible, try to ensure that vital information is available in text format on your site, so that everyone can read it.